Canister purge systems are generally used in the automotive industry to reduce automotive evaporative emissions by controlling the amount of fuel vapor released into the atmosphere from a vehicle's fuel supply, such as a fuel tank. Fuel vapor from the fuel supply is guided to and trapped in a collection canister containing a fuel vapor adsorbing material, such as activated carbon. The term "adsorbing" refers to the process of using solid particles to store fuel vapor, in comparison to the term "absorbing," which refers to the process of using a liquid to store fuel vapor. Fuel vapor is drawn out of the adsorbing material and into a low pressure intake chamber of the engine through a passage containing a purge control valve.
The purge control valve is normally closed, whereby the fuel vapor is retained within the canister. Periodically, when canister purging is required, the purge control valve is driven to an open position through application of an appropriate control signal to an actuator mechanically linked to the purge control valve. When the purge control valve is open and the engine is running, ambient air passing through a canister vent valve and across the canister to the relatively low pressure engine intake chamber draws fuel vapor from the fuel vapor adsorbing material into the engine intake chamber for ingestion in the combustion process.
Purge control strategies have been proposed in which the canister purge system is purged irrespective of system operating conditions. Such strategies can lead to purge system deterioration. For example, purging an empty canister contributes to purge system component wear and reduced purge system component life. Similarly, purging during high humidity conditions, such as when the air contains water molecules or water mist, draws moisture through the vent valve into the canister. Moisture in the canister can deteriorate the working capacity of the fuel vapor adsorbing material within the canister.
It would therefore be desirable to adjust canister purging under operating conditions known to lead to canister purge system deterioration. For example, it would be desirable to adjust or deactivate canister purging during periods of high humidity to prevent moisture intrusion and deterioration of the fuel vapor adsorbing material. Similarly, it would be desirable to adjust or deactivate canister purging when there is little or no vapor contained within the canister, preventing unnecessary purge system component wear and extending purge system component life.